About one-half of oil recovery worldwide is produced by waterflooding. Typically, more than one-half of the original oil in place remains in the reservoir after waterflooding and is the target for additional oil recovery processes. For reservoirs having been waterflooded to where the fraction of oil in the produced water is low, for example, less than 1%, keeping the well in production may only be barely economical. Such wells are commonly described as stripper wells. A large fraction of current production in the contiguous United States comes from stripper wells. For practical purposes the quantity of residual oil, often referred to as trapped oil, around the well bore is almost stable.
The injection of discrete volumes of fluid in a reservoir recovery process application is known. For example, in the Water Alternating Gas (WAG) process, one tenth of the reservoir hydrocarbon volume may be injected over a period of a year with cycles of one month of carbon dioxide injection followed by water injection for one month. However, significant quantities of oil remain in the reservoirs after primary and secondary recovery.
Laboratory studies have shown that when oil is recovered by waterflooding and at least a portion of the swept volume is recharged with oil, higher recovery is obtained when the recharged volume is swept by water a second time. Further cycles of recharging with oil and waterflooding may lead to yet additional oil recovery. Data for four sequential waterfloods leading to improved oil recovery have been reported (See, e.g., “Oil Recovery by Sequential Waterflooding with Oil Reinjection and Oil Relocation,” by Norman R. Morrow et al., International Publication No. WO 2009/126663, 15 Oct. 2009.), the disclosure and teachings of which are hereby incorporated by reference herein.
Measurement of residual oil and reduction in residual oil saturation has been demonstrated using the single well tracer test (SWTT) (See, e.g., McGuire et al. SPE 2005.). The SWTT for determination of residual oil typically investigates a region having a radius of 15 to 30 feet. The test involves injection of about 50 to 500 bbls of an aqueous solution containing the tracer. The actual volume generally depends on how much material can be injected in one day as determined by reservoir properties. This criterion is known from experience to give the best results for the SWTT. The tracer partitions between the residual oil and the aqueous phase, and the residual oil may be determined from separation between the partitioning tracer and a second tracer that remains in the aqueous phase (See, e.g., J. F. Tomich et al., “Single-Well Tracer Method to Measure Residual Oil Saturation,” Journal of Petroleum Technology (February 1973) 211-218.).